feat: Add LED indicator support (#3239)

This adds a new zmk,indicator-leds device, which maps HID indicator
states onto any devices that implement the LED driver API. This adds
support for things like a caps lock LED.

The name was chosen so that more drivers could be added later as
zmk,indicator-*, for example a version that uses the LED strip API.

app/src/indicators/Kconfig.indicator_leds

@@ -0,0 +1,19 @@
+# Copyright (c) 2025 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+config ZMK_INDICATOR_LEDS
+    bool
+    default y
+    depends on DT_HAS_ZMK_INDICATOR_LEDS_ENABLED
+    select LED
+    select ZMK_HID_INDICATORS
+
+if ZMK_INDICATOR_LEDS
+
+config ZMK_INDICATOR_LEDS_INIT_PRIORITY
+    int "ZMK indicator LED initialization priority"
+    default 91
+    help
+        System initialization priority for ZMK indicator LEDs.
+
+endif # ZMK_INDICATOR_LEDS